The maximum thermodynamic efficiency for the conversion of non-concentrated solar irradiance into electrical free energy for a single-band semiconductor absorber is approximately 31 percent [W. Shockley, H. J. Queisser, J., Appl. Phys., 32, 510 (1961)]. This efficiency is attainable in semiconductors with band gap energies ranging from 1.25 to 1.45 electronvolts (eV). For semiconductors, band gap generally refers to the energy difference between the top of the valence band and the bottom of the conduction band. The solar spectrum, however, contains photons with energies ranging from about 0.5 to about 3.5 eV. Photons with energies below the semiconductor band gap energy, Eg, are not absorbed. On the other hand, photons with energies above the band gap energy create charge carriers with a total excess kinetic energy, Ek(excess)=hν−Eg, where hν is the photon energy. A significant factor limiting the conversion efficiency to 31% is that the excess kinetic energy (absorbed photon energy above the semiconductor band gap energy) Ek(excess) is lost as heat through electron-phonon scattering. “Hot” electrons and holes that are created by absorption of solar photons with energies larger than the band gap energy will relax to their respective band edges.
For a single-band-gap semiconductor absorber there are two ways to extract energy from hot carriers before they relax to the band edge. One method produces an enhanced photovoltage, and the other method produces an enhanced photocurrent. The former method involves extraction of hot carriers from a semiconductor absorber before they relax to their respective band edges. Extracting energy from hot carriers, before they relax to the band edge, is possible if the relaxation rate of hot carriers to their respective band edges is slowed. In the latter method, hot carriers produce two or more electron-hole pairs—so-called impact ionization.
P-n junction solar cells are the most common solar cells, including a layer of n-type semiconductor in direct contact with a layer of p-type semiconductor. If a p-type semiconductor is placed in intimate contact with a n-type semiconductor, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). The diffusion of carriers does not happen indefinitely, however, because of an opposing electric field created by this charge imbalance. The electric field established across the p-n junction induces separation of charge carriers that are created as result of photon absorption.
Dye-sensitized solar cells and quantum dot-sensitized solar cells are two next generation solar technologies. In dye-sensitized solar cells, dye molecules are chemisorbed onto the surface of 10 to 30 nanometer (nm) size titanium oxide (TiO2) particles that have been sintered into high porous nanocrystalline 10 to 20 μm thick TiO2 films. Upon the photo excitation of dye molecules, electrons are injected from the excited state of dye into the conducting band of the TiO2 creating a charge separation and producing photovoltaic effect. The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an organic solvent containing redox system, such as the iodide/triiodide couple. It is generally accepted that, in dye-sensitized solar cells, the electron transport through the oxide is predominantly governed by diffusion, because the highly conductive electrolyte screens the interior of the cells from any applied electric field.
In quantum dot-sensitized solar cells, semiconductor particles with sizes below 10 nm (so-called quantum dots) take the role of the dye molecules as absorbers. In these solar cells, the hot carriers may produce two or more electron-hole pairs, so-called impact ionization, increasing efficiency of these solar cells. Quantum dot-sensitized solar cells offer several other advantages. The band gap energies and thereby the absorption ranges are adjustable through quantum dot size or composition. Furthermore, compared to organic dyes, quantum dot sensitization offers improved stability, since the surface of the semiconductor quantum dot can be modified to improve its photostability.
A noted drawback of both dye-sensitized and quantum dot-sensitized solar cells is long term stability due to the presence of electrolyte. In order to improve stability of quantum dot-sensitized solar cells, the redox electrolyte in these cells can be replaced with a solid hole-conducting material, such as spiro-OMeTAD, or a p-type semiconductor. The former is called a solid state dye-sensitized solar cell, if an absorber is a dye molecule, or a solid state quantum dot-sensitized solar cell, if an absorber is a quantum dot. The latter solar cell, including a p-type semiconductor, is called an extremely thin absorber (ETA) solar cell. In this solar cell, a porous nanocrystalline TiO2 film is covered with a p-type semiconductor absorber using an atomic layer deposition technique, or using electrochemical deposition. These techniques enable a conformal deposition of a semiconductor on top of TiO2. A p-type semiconductor first fills up the pores of porous TiO2 film and then tops the whole structure with a layer about 10 to 200 nm thick. Because of the rough TiO2 surface and a conformal deposition of a p-type semiconductor, the interface area between a p-type semiconductor and an oxide layer increases more than 10 times in comparison to that for a flat TiO2 film covered by a p-type semiconductor layer.
To decrease the relaxation rate of charge carriers, an absorber semiconductor can be inserted between TiO2 (n-type semiconductor) and the solid hole conductor material. In this structure, the n-type semiconductor (oxide, example: TiO2) has a porous structure and the absorber semiconductor is adsorbed at the surface of n-type semiconductor forming individual quantum dots. The average size of the absorber semiconductor quantum dots is below 10 nm to utilize the confinement effect and reduce the relaxation rate of hot carriers increasing efficiency of these solar cells. In the existing fabrication processes of solid state sensitized solar cells porous or rough TiO2 layer is filled (using-electrochemical deposition techniques) with absorber semiconductor grains and covered with p-type semiconductor (using atomic layer deposition or electrochemical deposition techniques) or a different hole conducting inorganic material (using for example spin coating of a solution of hole conductor and chlorobenzene (See J. Kruger, U. Bach, R. Plass, M. Piccerelli, L. Cevey, M. Graetzel, Mat. Res. Soc. Symp. Proc., 708, BB9.1.1 (2002)).
The hole conductor may be an organic transport material. This organic charge transport material may be a polymer, like poly-tiophen or poly-arylamin. The hole conductor may be an organic hole conductor such as spiro- and hetero spiro compounds of the general formula (1)
where φ is one of C, Si, Ge or Sn, and K1 and K2 are, independently one of the other conjugated systems. One example organic hole conductor is spiro-OMeTAD (2,2′,7,7′-tretakis(N,N-di-p-methoxyphenyl-amine)-9-9′-spirobifluorene). The conductivity of pure spiro-OMeTAD is low. Therefore the material cannot be used, without some modification, in solar cells. Rather, partial oxidation of spiro-OMeTAD by N(PhBr)3SbCl6 can be used to control the dopant level and to increase the conductivity of the hole conducting layer. A second additive Li[CF3SO2]2N can also be added, since Li+ ions have been shown to increase the current output and overall efficiency of the device. The hole conductor matrix can be applied by spin-coating of a solution of the hole conductor in chlorobenzene. MEH-PPV [poly[2-methoxy-[5-(2′-ethyl)hexyl]oxy-p-phenylenevinylene]] and PEDOT:PSS [poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)] can also be used as hole conductor materials. To increase its conductivity PEDOT:PSS can be mixed with glycerin, N-methylpyrrolidone, and isopropanol.